1. Field of Invention
The present invention relates to the measurement of chromatic dispersion characteristics of a DUT (Device Under Test) such as an optical fiber, and in particular to the measurement which can obtain synchronization of a variable-wavelength light source and a phase comparator with a high precision, by providing the variable-wavelength light source at one end of the DUT and the phase comparator at the other end of the DUT.
2. Description of the Related Art
The construction of a measuring system which measures chromatic dispersion characteristics of the DUT such as an optical fiber is shown in FIG. 8. A light source system 100 is connected to one end of an optical fiber 300 and a measuring system 200 is connected to the other end of the optical fiber 300. The light source system 100 has a variable-wavelength light source 102 and an optical modulator 104. The measuring system 200 includes a photoelectric (OLE) converter 202 and a phase comparator 204.
In measuring chromatic dispersion characteristics, the variable-wavelength light source 102 changes the wavelength xc3xax of generated light. The light generated by the variable-wavelength light source 102 is modulated by a modulation frequency Fm in the optical modulator 104 and inputted to the optical fiber 300. The light transmitted through the optical fiber 300 is converted into an electric signal in the photoelectric (OLE) converter 202. The phase comparator 204 measures a phase difference between a phase of an electric signal and a phase which is to be a reference with respect to the electric signal. Group delay (GD) can be calculated from the phase difference and modulation frequency Fm. Chromatic dispersion (CD) can be calculated by differentiating group delay by the wavelength of the group delay. In addition, the frequencies of xc3xax and Fm are communicated to the measuring system 200.
Waveforms of light generated by the light source system 100 and light received by the measuring system 200 are schematically shown in FIGS. 9(a)-9(b). FIG. 9(a) shows the waveform of light generated by the light source system 100. FIG. 9(b) shows the waveform of light received by the measuring system 200. Time delay t0 added to the light generated by the light source system 100 makes a light to be received by the measuring system 200. However, for simplicity, the drawing shows as if there is no discrepancy of phases between the light generated by the light source system 100 and the light received by the measuring system 200. The time delay t0 is L/(c/n) {t0=L/(c/n)}, where L is length of optical fiber 300, c is velocity of light, and n is refraction index of optical fiber 300. In addition, t0 is increased as the length of optical fiber increases. For example, the length of optical fiber in a submarine cable and the like is about 10000 km, and to is up to 50 ms.
As shown in FIGS. 9(a)-9(b), in the light received by the measuring system 200, the time delay t0 is generated. Therefore, if the light source system 100 changes a wavelength directly after a light of a certain wavelength is generated, it becomes impossible to know xc3xa x (wavelength of light generated by the variable wavelength light source 102) corresponding to the light received by the measuring system 200.
Therefore, the wavelength xc3xa x of light generated by the light source system 100 is fixed from t0 to t1. FIG. 10 shows a method for changing the wavelength of light generated by the light source system 100. Firstly, a light, the wavelength of which is xc3xa 0 from time 0 to t1, xc3xa 1 from time t1 to 2t1, and so on, is generated. That is, the wavelength of light is changed in a step form.
Here, the variable-wavelength light source 102 cannot perform measurement of wavelength while continuously changing the waveform, even if it had a function which renders it possible to continuously sweep the wavelength. This is because it is impossible to exactly known xc3xa x (wavelength of light generated by the variable-wavelength light source 102) corresponding to the light received by the measuring system 200. That is, it is impossible to obtain a synchronization of light source system 100 and measuring system 200. Therefore, the wavelength of light is changed in the step form and measured.
However, if the wavelength of light is changed in the step form and measured, the time required for measuring is longer than that required in the case of continuously sweeping the wavelength. Moreover, if wavelength changing values (xcex2-xcex1, xcex1-xcex0, . . . ) are not taken so high to a certain extent, the measuring time takes too long. Therefore, it is impossible to improve the resolution of wavelength.
Therefore, the object of the present invention is to provide a technique for measuring characteristics, such as chromatic dispersion and the like, by making it possible to continuously sweep the wavelength of light source.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; an optical modulation unit for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test; and an identification waveform detection unit for detecting the identification waveform in the transmitted light transmitted through the device-under-test.
According to the apparatus for measuring optical characteristics constructed as explained above, since the time when the identification waveform detection unit detects the identification waveform is the time when the waveform starts to change, it is possible to obtain a synchronization between an incidence side and an exit side of a device-under-test using the time when the identification waveform is detected. Accordingly, it is possible to obtain the synchronization between an incidence side and an exit side of a device-under-test, even if the wavelength of light source is continuously swept.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; and an optical modulation unit for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: an identification waveform detection unit for detecting identification waveform in a transmitted light which is an incident light transmitted through the device-under-test, wherein the incident light is a variable-wavelength light, the wavelength of which is variable, having in the form of the identification waveform at the time when the wavelength is changing, and wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing.
The present invention described above, is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in wavelength.
The present invention described above, is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in output condition.
Here, the output condition corresponds to ON or OFF of the light source.
The present invention described above, is an apparatus for measuring optical characteristics, wherein the device-under-test includes a first optical line that transmits light only in one direction, and a second optical line that transmits light only in a direction opposite to the one direction, and wherein the variable-wavelength light source and optical modulation unit are connected to the incidence side of the first optical line and the identification waveform detection unit is connected to the exit side of the second optical line.
According to the present invention, an apparatus for measuring optical characteristics, further includes: a phase measuring unit for measuring the phase of the transmitted light by correlating it with the wavelength of the variable-wavelength using the time at which the identification waveform detection unit detects the identification waveform; and a characteristic calculation unit for calculating group delay characteristics or dispersion characteristics of device-under-test using the phase of the transmitted light.
According to the present invention, a method for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light generating step for generating a variable wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; an optical modulation step for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test; and an identification waveform detection step for detecting the identification waveform in the transmitted light transmitted through the device-under-test.
According to the present invention, a method for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light generating step for generating a variable- wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; and an optical modulation step for modulating the variable-wavelength light to a predetermined frequency and then inputting it to the device-under-test.
According to the present invention, a method for measuring optical characteristics of a device-under-test which transmits light, includes: an identification waveform detection step for detecting an identification waveform in a transmitted light which is an incident light transmitted through the device-under-test, wherein the incident light is a variable-wavelength light, the wavelength of which is variable, having in the form of the identification waveform at the time when the wavelength is changing, and wherein the identification waveform is distinguishable from a norinall waveform before and after the time when the wavelength is changing.
The present invention, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring a characteristic of a device-under-test which transmits light, the optical characteristic measuring process including: a variable-wavelength light generating processing for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and, after the time when the wavelength is changing; an optical modulation processing for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test; and an identification waveform detection processing for detecting the identification waveform in the transmitted light transmitted through the device-under-test.
The present invention, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring a characteristic of a device-under-test which transmits light, the optical characteristic measuring process including: a variable-wavelength light generating processing for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; and an optical modulation processing for modulating the variable wavelength light at a predetermined frequency and then inputting it to the device under-test.
The present invention, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring a characteristic of a device-under-test which transmits light, the optical characteristic measuring process including: an identification waveform detection processing for detecting identification waveform in a transmitted light which is an incident light transmitted through the device-under-test, wherein the incident light is a variable-wavelength light, the wavelength of which is variable, having in the form of the identification waveform at the time when the wavelength is changing, and wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing.
The present invention is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in wavelength.
The present invention, is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in output condition.
The present invention, is an apparatus for measuring optical characteristics, further including: a phase measuring unit for measuring the phase of the transmitted light by correlating it with the wavelength of the variable-wavelength using the time at which the identification waveform detection unit detects the identification waveform; and a characteristic calculation unit for calculating group delay characteristics or dispersion characteristics of device-under-test using the phase of the transmitted light.